The invention concerns a directional microphone.
Modern hearing devices resort to directional microphone arrangements that, via their direction-dependent microphone sensitivity, enable an exclusion of unwanted signals coming from lateral and backwards directions. This spatial effect improves the wanted-signal-to-background-noise ratio, such that, for example, an increased speech comprehension of the wanted signal exists. The conventional directional microphone arrangements are based on an evaluation of the phase (delay) differences that result given a spreading sound wave between at least two spatially separate sound acquisition locations.
In hearing devices, until now, gradient microphones or, respectively, directional microphone arrangements of a first and higher order, comprising a plurality of omnidirectional acoustic pressure sensors, have been used for this. While the first determines the difference (stemming from the mechanical assembly) of the sound signals originating from two sound entrance ports, a good static or even adaptively variable directional effect can be achieved via suitable signal processing, given a combination of a plurality of acoustic pressure sensors.
However, all known methods evaluate the differences of the sound signals present at the sound entrance ports in the same manner. Since the distances between the sound entrance ports in hearing device applications are very small (conditional upon the type), this leads to the fact that, given deeper frequencies at which the sound wavelength is much larger than the separation of the microphone entrance ports, the differences to be determined between the audio signals, and thus also the directional effect to be achieved, are very small. Typically, all directional microphone arrangements possess a clearly reduced directional effect at lower frequencies; moreover, arrangements made up of a plurality of pressure sensors place very high demands on the amplitude and phase compensation of the microphones.
A differential pressure transducer is known from U.S. Pat. No. 4,974,117 that capacitively couples two membranes, where the pressure difference is measured between the pressure in the volume between the membranes and the pressure in the volume that surrounds both membranes.
In imitation of the acoustic organ of the “Ormia” fly, which achieves a unique directional effect with the aid of a mechanical coupling of two auditory membranes, various approaches to use mechanically coupled auditory membranes in hearing aid devices have been pursued. For example, in a microphone system based on silicon micromechanics, the vibration-capable membrane of two independent microphones arranged adjacent to one another are negatively coupled with one another via a web (see “Mechanically Coupled Ears for Directional Hearing in the Parasitoid Fly Ormia Ochracea”, R. N. Miles, D. Robert, R. R. Hoy, Journal of the Acoustical Society of America 98 (1995), pg. 3059).